Generally, it is common to use a water phantom for measuring radiation dose outputted from radiation devices such as X-ray generators, Co-60 therapeutical apparatuses and linear accelerators, as well as the quality control regarding the same, by which the radiation dose generated from those radiation devices can be calibrated for enabling the same to conform with standard dose for medical treatments, and the same time that as their output radiation characteristics can be evaluated. With respect to its functionality, the water phantom is featuring in that: a good water phantom should be designed with a specific volume that is large enough for sustaining a scattered radiation in a saturated manner, and moreover, it should be able to maintain a stable water level so that the thickness measured between an inspection point submerged in the water phantom and its water level can be maintained without changing with the undulation of the water level and thus prevent the dose measurement from being interfered by the undulation of water level.
Please refer to FIG. 1, which shows a conventional water phantom. The water phantom of FIG. 1 is comprised of: a tank 11, a water solution 12, a mobile rack 13, and an ionization chamber 14. The mobile rack 13 as well as the ionization chamber 14 attached thereto can be driven to move by a motor. It is noted that the reference level for measurement in the aforesaid water phantom is its water level, and it has the following shortcomings:                (1) As the mobile rack 13 can be driven to move in the tank 11 in a three-dimensional manner, i.e. it can be driven to move up, down, front, back, left or right in the tank 11, the movement of the mobile rack 13 will cause the water level of the water solution 12 to undulate and thus further cause the depth of the ionization chamber 14 to vary accordingly in a continuous manner during a radiation measurement so that the accuracy of the measurement will be adversely affected.        (2) As the reference level of the water solution 12 can be affected by its surface tension that it is difficult to define and measure, the depth of any location submerged in the water solution 12 is difficult to obtained and can be erroneous.        (3) As the raising/descending of the mobile rack 13 will cause the reference level of the water solution 12 to changed accordingly, the radiation measurement will be interfered so that measurement may be erroneous.        
Therefore, it is in need of a water phantom structure with fixed water level for overcoming the aforesaid shortcomings.